The discovery of visible photoluminescence (PL) and electroluminescence (EL) from porous silicon has stimulated significant interest in this material and other nanoporous materials. Efficient visible luminescence may be achieved in porous semiconductor layers (e.g. silicon, germanium, silicon carbide, etc.), which has significant economic potential in optoelectronic devices (such as efficient visible emitters, solar cells, photodetectors, photonic band-gap crystals, displays, etc.), in gas and chemical sensors, and as sacrificial layers to realize 3-D patterns with high aspect ratio on bulk semiconductors.
Luminescent porous materials are currently made by a number of methods, including electrochemical anodization, chemical stain etching, hydrothermal etching and spark erosion techniques. In addition, lasers, ion beams and electron beams have also been used to modify the surface properties of various materials such as semiconductor materials.
Although porous materials can be produced by electrochemical anodization and spark erosion techniques, control of such processes is complicated. Using these techniques, it is also very difficult to make nanoporous materials from non-conductive substrates. A good electrical contact must first be formed and then it must be protected during the entire electrochemical etching process. When an electric current through a substrate is used, it is almost impossible to define areas of preferential etching, which makes it difficult for large scale integration (LSI).
When applied to silicon, anodic etching is limited to certain types of doped silicon. The process is difficult to control, particularly for n-type structures, and is not compatible with standard silicon fabrication technology. The formation of patterns is restricted by the application of current to the entire substrate. It is difficult to selectively form a high resolution pattern on the surface of the substrate.
Chemical stain etching is more suitable for massive industrial productions, but, when it is used alone the depth of the etching is shallower than electrochemical anodization. Also, the wetting period is relatively long for chemical stain etching alone, rendering the morphology of the resulting porous material rough and irregular. When used alone, chemical stain etching is usually slow (characterized by an induction period), irreproducible, unreliable in producing light-emitting porous materials, and is mainly used for making very thin layers.
There still remains a need in the art for simple and effective processes for producing luminescent porous materials.